1. Field of the Invention
The present invention relates to a gas-flow measuring apparatus which is used, for example, for measuring the flow rate of engine intake air.
2. Description of the Prior Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important driving parameters for controlling the base fuel injection amount, base ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter, is provided for this purpose.
One of the more common prior art airflow meters is the hot-wire type. For example, the hot-wire-type airflow meter may consist of an electric heater provided in an intake-air passage of an engine and temperature-dependent resistors arranged before and after the electric heater, the electric heater and the temperature-dependent resistors being made of platinum wires of the same characteristics floating in the gas stream. In this type of airflow meter, however, the hot-wires are easily broken due to vibration, shock, and the like, especially due to engine backfires.
In order to generate a voltage corresponding to the flow rate of air, two fixed resistors are connected to the temperature- dependent resistors, thereby forming a bridge circuit. The difference in potential between the diagonal points of the bridge circuit is amplified and fed back to the electric heater. In this case, however, it is necessary to remarkably reduce the difference in temperature coefficient between the fixed resistors and the temperature-dependent resistors. For this purpose, additional steps for adjustment are necessary, increasing the manufacturing cost.